1. Field of the Invention
The present invention relates to a polymerization process of acrylamide and, more particularly, it relates to an improved polymerization process of acrylamide wherein an aqueous solution of an acrylamide monomer in a comparatively high concentration can be easily polymerized by substantially stopping or inhibiting the occurence of hydrolysis of the acrylamide polymer caused by the temperature increase of the reaction system by heat of the polymerization and the unavoidable heat aging in the course from the end of the polymerization to the completion of the cooling operation in an industrial production.
2. Description of the Prior Art
Acrylamide polymers have been widely utilized as soil stabilizers, flocculants for water treatment, mucilage-like materials for paper manufacturing, and other water-soluble polymers. In particular, various utilizations of water-soluble acrylamide polymers have recently been studied, for example, as flocculants, etc., and in using as the flocculant for waste water from paper manufacturing, polyacrylamide which has a high molecular weight and has not been hydrolyzed is frequently required.
The polymerization of acrylamide can be conducted as an aqueous solution polymerization, a suspension polymerization, a solution polymerization, or a block polymerization by a radical initiator, the action of light, ultrasonic waves, high energy radiation, heat, etc., but aqueous solution polymerization has been most widely employed since a polymer having a high molecular weight can be easily obtained, since the monomer used and the polymer produced are soluble in water, and also since the product can be obtained in low cost.
On the other hand, on considering that the use of acrylamide polymers in the form of a powder has recently increased in the fields of flocculants and others, the polymerization of acrylamide in a low concentration which has hitherto been employed frequently is undesirable where the product is to be powdered by drying and a polymerization of acrylamide in a concentration as high as possible has been desired.
However, the viscosity of an aqueous solution of polyacrylamide increases as the molecular weight of the polyacrylamide increases. The concentration of the aqueous solution of polyacrylamide which can be handled as a liquid, that is, a solution which can be transported by pumping and which can be stirred, is up to about 10% by weight of a high molecular weight polyacrylamide which can be used satisfactorily as a flocculant. Thus, it is industrially quite difficult to handle an aqueous solution of polyacrylamide having a concentration higher than about 10% by weight as a liquid.
In order to overcome these difficulties, an aqueous solution of acrylamide at a high concentration is polymerized to provide a massive (gelatinous) product rather than a polymer solution, whereby the product can be handled as a solid, i.e., in transportation, etc., thereof and further the energy required to dry the product can be reduced.
However, on the other hand, on considering the polymerization reaction of acrylamide, the polymerization of an aqueous solution of an acrylamide monomer at a concentration of higher than about 10% by weight, for example, of about 20% by weight is accompanied by the formation of a massive or gelatinous product. The gelatinous character of the product makes it difficult to stir the polymerization system and thus to remove the heat of polymerization. In other words, the same result as in an adiabatic polymerization are obtained in the above described polymerization reaction. Thus, the heat of polymerization of acrylamide, 19.8 .+-. 0.7 Kcal/mol becomes a heat source increasing the temperature of the polymerization system and the heat generated due to the exothermic reaction, as a matter of course, increases as the concentration of the acrylamide monomer in the polymerization increases. The practical temperature increase in the polymerization system is as follows. That is, if the polymerization of acrylamide is carried out at a starting temperature of 25.degree.C in an apparatus which is so constructed that heat does not escape therefrom, the maximum temperature of the polymerization system increases to about 40.degree.C, about 55.degree.C, about 70.degree.C, about 85.degree.C and about 100.degree.C as the concentration of the acrylamide monomer increases to 5%, 10%, 15%, 20%, and 25% by weight, respectively.
In polymerizing an aqueous solution of acrylamide at a comparatively high concentration, the polymerization system becomes a near solid state and thus the removal of the heat of polymerization by stirring becomes difficult. This results in increasing the temperature of the polymerization system and causing a hydrolysis of the polymer produced.